Duchenne Muscular Dystrophy (DMD) in both humans and dogs is a fatal, X-linked, recessive muscle disease caused by lack of dystrophin due to deletions or mutations in the dystrophin gene. Adeno-associated virus (AAV)-mediated delivery of micro-dystrophin to skeletal muscle has been successful in mice;however, recent studies indicate that the efficacy of AAV-mediated therapies might be limited by an immune response to viral capsid proteins in humans. By direct intramuscular injection of AAV vectors in wild type and cxmd (canine X- linked muscular dystrophy) dogs, we demonstrated robust cellular immune responses to AAV capsid proteins, suggesting the likelihood of cellular immunity to AAV vectors in humans. We further demonstrated that the immune response generated following intramuscular injection of AAV vectors could be averted by a brief course of intense immunosuppression. The broad, long-term objective of this project is to develop AAV- mediated gene therapy strategies in cxmd dogs that can be applied to human patients with DMD. In this revision application, we propose to test novel AAV vectors that appear to be much less immunogenic than current vectors. These vectors have been developed by Drs. Miller and Halbert at the FHCRC and are based on their finding that current AAV vectors can transfer and express AAV capsid genes, and that transfer can be avoided by modification of the vector production system. Here we propose to test the hypothesis that these novel vectors will significantly reduce immune responses in normal and cxmd dogs leading to a reduced need for immunosuppression. These studies will provide necessary information for future human trials of AAV- mediated gene therapy in humans with Duchenne muscular dystrophy. PUBLIC HEALTH RELEVANCE: The significance of this revision application is that the methodologies developed in the canine X-linked muscular dystrophy (cxmd) model can be used to treat human patients with Duchenne muscular dystrophy (DMD). Reducing the immunogenicity of AAV vectors and developing better and less toxic immunosuppression regimens will increase the likelihood of achieving the goal of effective gene therapy for human DMD.